216 
PROFESSOR W. 0. WILLIAMSON ON THE ORGANIZATION 
sentation of the part of the central cylinder enclosed between the points fig. 6, b\ b'. At 
fig. 19, V we find a cluster of the vessels bending outwards from their normal vertical course. 
In fig. 7, b'* we find that a further outward extension of these divergent vessels has taken 
place. Fig. 20 is an enlargement of this part of the cylinder. In it we see that the 
vessels which bend outwards in the direction of the arrow through a part of their 
course ( b ") are again bending upwards at b\ so as to be intersected transversely. In fig. 8, V 
we find the bundle completely detached from the central vascular axis, and now it is evi- 
dently a bundle like c of fig. 2, only located on the opposite side of the stemf. In 
figs. 11 & 12 we have manifest evidence that this bundle V is about to follow the course 
of its predecessor on the opposite side of the stem, since the outer bark (f) is rapidly 
approaching to the conditions seen at f in the figures 8 & 9. 
Fig. 21 exhibits a somewhat oblique longitudinal section which passes through the 
central cylinder at b, and is nearly in the plane of the primary bundle c, the sheat 
of the latter being seen at g. This little section is important, since it shows that 
the ascending direction followed by this primary bundle is also followed by certain 
smaller ones yet to be described, as at fig. 21, cl. 
Where the vessels are about to leave the central cylinder to form one of the primary 
bundles just described, as at fig. 19, b', several of my sections reveal a curious change in 
the structure of some of the elementary tissues. As already stated, the vessels composing 
both the central cylinder and the primary petiolar bundle are of the barred type so 
common amongst the Carboniferous plants. Fig. 22 exhibits a cluster of the tissues 
seen at fig. 19, b', enlarged 62 diameters. At b we see barred cells which, yet further 
to the right of the section, pass into the ordinary barred vessels of the axis ; but at V 
we find similar cells, but with reticulate instead of barred walls, and at b 11 similar 
reticulated structures are being elongated, approaching the form of the ordinary vessels 
of the plant. This conversion of the barred into the reticulate type, the latter being a 
mere modification of the former, is merely a local phenomenon. I only detect it where 
the central cylinder is about to give off a petiolar branch, and here only in a very limited 
measure ; no trace of reticulate vessels is to be found elsewhere, either in the central or 
the lateral bundles. 
Secondary bundles. — In addition to the large vascular bundles supplying the petioles 
we find in several of the sections smaller bundles, some of which are given off directly 
from the central cylinder, others from the primary petiolar bundles. Instances of the 
former of these orientations are found in figs. 3, 5, 7, 10, & 12, and an example of the 
latter occurs in fig. 5. In fig. 3 a bundle is separating from the upper part of the 
* The arrows of the figures 7 and 20 indicate the directions in which these two figures should be turned to 
place them in identical positions. 
f In figs. 8 & 9, 10, 11, & 12 the sections have been slightly twisted round to make them fit more 
symmetrically upon the plate ; hence, whilst the vessels V of fig. 7 point downwards towards the lower right- 
hand corner of the section, in all the subsequent sections referred to the bundle which these divergent vessels 
have formed is situated directly to the right of the cylinder. 
